1. Field of the Invention
This invention relates to energy absorbing rigid polyurethane foam compositions and their methods of preparation. Specifically, the energy absorbing rigid polyurethane compositions of this invention are water blown and employ lithium salts to promote cell opening, enabling the foam to exhibit minimal spring back or hysteresis. Such foams are suitable as lightweight alternatives to traditional energy absorbing applications, such as side impact bolsters in automobile doors and foam blocks for floral arrangements.
2. Description of the Related Art
Recent years have seen an accelerated growth in the field of energy absorbing foams, especially in the automotive industry. Heightened safety concerns over the safety of passengers has generated numerous federal safety standards, among which are passive restraint systems such as air bags. As pointed out in "Fundamental Studies of Polyurethane Foam for Energy Absorption in Automotive Interiors" by J. A. Thompson-Colon, et al. in SAE Technical Paper Series 910404, the advent of air bags has required the automotive industry to look to the manufacture of energy absorbing instrument panels, or knee bolsters, since one tends to slide under a deployed air bag and impact the knee on the panel. Also, currently under study is the use of energy absorbing foams as hip or shoulder bolsters to protect the hip and shoulder regions of a passenger or driver against impacts during collisions to the side of the car.
One of the requirements of a good energy absorbing foam is that it is open celled and exhibits a constant or nearly constant compressive strength at deflections ranging from about 10 percent to about 60 percent. Upon impact, the cell struts and walls crush and thereby dissipate the energy of the impacting object. Air trapped within a closed-celled foam, however, imparts structural strength to the walls and struts upon compression resulting in a resilient foam that exhibits an exponential increase in compressive strength upon continued deflection through the foam. Thus, it is desirable to open the cells of the foam as much as possible.
As is discussed in detail in the description of the invention, it has now been found that the use of lithium salts of an organic acid apparently open the cells of rigid polyurethane foams. The use of lithium salts as catalysts in CFC-blown rigid polyurethane foams is known from such U.S. Pat. No. 4,107,069, which describes a lithium, sodium or potassium salt of a 2-20 carbon atom carboxylic acid chain as a gel catalyst to preserve the reactivity of a CFC-containing masterbatch for rigid polyurethane foams; U.S. Pat. No. 4,256,847 which describes a mixture of lithium and zinc salts to catalyze rigid foam systems blown by CFCs with the sole use of lithium salts as catalysts discouraged due to their high catalytic activity; U.S. Pat. No. 3,108,975 which describes a mixture of alkali metal hydroxides and alkali metal salts of acids as a catalyst for the production of polyurethane foams blown with high quantities of water, the only examples being that of flexible foams having resiliency and the absence of the hydroxide failing to produce a foamed product; U.S. Pat. No. 3,041,295 which describes a chlorinated phosphate ester containing polyurethane foam (flexibles exemplified) blown with high quantities of water by incorporating a lithium salt into the prepolymer to preserve the foam against humidity breakdown; U.S. Pat. No. 3,634,345 which describes the use of alkali metal salts of o-hydroxycarboxylic acids as a catalyst to polymerize the isocyanate into isocyanurate rings in a coating, elastomer, or foam blown with 5 to 50 parts CFC and optionally water; U.S. Pat. No. 3,769,245 which describes the use of alkali metal salts of carboxylic acids to catalyze the reaction of a dicarboxylic acid with the isocyanate group to release carbon dioxide and produce a thermoplastic polyurethane foam blown in the absence of water; U.S. Pat. No. 3,940,517 which describes alkali metal salts as catalysts in the production of polyisocyanurate foams blown with CFCs, and U.S. Pat. No. 5,084,485 which describes using an alkali metal carboxylate (only potassium acetate mentioned) as a trimerization catalyst in an isocyanurate foam blown with water to yield a closed cell insulation board. In each of these patents, however, the alkali metal salts are employed as catalysts; and none describe energy absorbing properties of a foam. The above patents also describe the production of CFC-blown foams, large water content blown foams, high density foams, resilient foams, predominately polyisocyanurate and closed celled foams, or thermoplastic foams.
Examples of described energy absorbing foams are found in various patents and publications. U.S. Pat. No. 4,866,102 describes moldable energy absorbing rigid polyurethane foam compositions which are prepared by the reaction of a graft polymer dispersion in a polyoxyalkylene polyether polyol with an alkylene oxide adduct of toluenediamine or diaminodiphenylmethane with an organic polyisocyanate in the presence of a crosslinking agent and a chlorofluorocarbon (CFC) blowing agent. Other patents describe energy absorbing foams which are flexible or semi-rigid, are resilient, have utility in bumper cores, and have molded densities in excess of 5 pcf, such as U.S. Pat. Nos. 4,190,712, 4,116,893; 4,282,330; and 4,212,954. The foams described in these patents, although employing the phrase "energy absorbing," are not useful for the purposes of this invention since they exhibit resiliency or recovery. The foams of this invention are rigid and crush upon impact, exhibiting little or no rebound, and preferably have molded densities of less than 2.8 pcf. U.S. Pat. No. 4,722,946 describes the production of energy attenuating viscoelastic polyurethane elastomers and foams, rather than rigid foams, comprising mixtures of linear and branched polyol intermediates, polyisocyanates, and optionally, extenders, blowing agents, and the like, in the presence of a catalyst whereby the isocyanate index is varied from about 65 to about 85. U.S. Pat. No. 4,664,563 describes a method of shoring a geological formation which comprises preparing a high density (19 pcf-50 pcf) rigid polyurethane foam having a specific oxyalkylated toluenediamine as the polyol, which exhibits nearly constant strain with increasing stress in compression. Similarly, U.S. Pat. No. 4,614,754 described a high density (&gt;17 pcf) rigid polyurethane foam which exhibits nearly constant strain with increasing stress in compression at high loadings (&gt;600 psi) by reacting a specific alkoxylated toluene diamine. Again, the foam must be prepared by using a specific polyol; and foams with such high densities and high loadings at yield are not usable for the automotive interior applications described above or as floral foams. U.S. Pat. No. 4,696,954 describes the preparation of high density (&gt;25 pcf) molded polyurethane foams blown with CFCs characterized by high impact strength and good thermal stability.